1. Field of the Invention
The invention relates to multiple-input-multiple-output (MIMO) communication systems and methods of operating MIMO systems.
2. Discussion of the Related Art
A MIMO communication system includes a transmitter with multiple transmitting antennas, a receiver with multiple receiving antennas, and a free space channel coupling the transmitting and receiving antennas. The transmitting antennas transmit on the same frequency band in each time slot. For that reason, a transmitter having M transmitting antennas can be viewed as sending a row of M signal values in each time slot. If the transmitter transmits a message over T time slots, the message is associated with a T×M dimensional matrix, U, of signal values.
The free space channel couples the multiple transmitting antennas to various ones of the receiving antennas. Thus, in each time slot, individual receiving antennas receive a signal combining transmitted signals from more than one of the transmitting antennas. A transmission matrix, H, whose elements are the complex channel attenuations between various pairs of transmitting and receiving antennas, defines these combinations. In T time slots, N receiving antennas will receive a message that is described by a T×N dimensional matrix Y. The matrix Y approximately satisfies Y=U·G·H+w where w is an additive noise matrix and G is a diagonal matrix of transmission gains.
Thus, knowledge of the transmission matrix, H, can enable the MIMO receiver to disentangle the signals transmitted by different ones of the transmitting antennas, e.g., if H is invertible. For that reason, it is sometimes desirable to measure the transmission matrix, H. One method of measuring H, involves transmitting standard pilot beams from various ones of the transmitting antennas and measuring the signals received in response to the transmission of the standard pilot beams. Unfortunately, the use of pilot beams is not convenient in all MIMO communication systems. For example, the uses of such pilot beams may not be enable measurements of the transmission matrix, H, for a channel whose properties are changing rapidly. In such MIMO communication systems, decoding is performed without a detailed knowledge of the transmission matrix.